The invention relates to a hydraulic brake system with slip control for automotive vehicles, with a pedal-operable braking pressure generator and with wheel brakes connected at the braking pressure generator, in which brake system between the braking pressure generator and the wheel brakes valve means are arranged which are controllable by slip control electronics and by means of which the connections to the wheel brakes may be locked and connections may be established between the wheel brakes and a storage reservoir.
A device with the above characteristics is described in the German published and printed patent application DE-OS No. 30 40 561.9. The prior art brake system has a brake booster followed by a master cylinder. At the master cylinder first brake circuits are connected which are preferably arranged at the front axle of an automotive vehicle. A second brake circuit is connected at the pressure chamber of the hydraulic power booster. The second brake circuit communicates with wheel brakes arranged at the rear axle of the automotive vehicle.
Electromagnetically operable valve means are connected in the connection between the master cylinder and the wheel brakes arranged at the front axle of the automotive vehicle. The electromagnetically operable valve means may be switched by slip control electronics into a locking position so that the pressure in the wheel brakes connected to the master cylinder may be kept constant independently of the pressure generated in the master cylinder. Further, at the wheel brakes arranged at the front axle, electromagnetically operable valve means are connected which normally are in a locking position and which upon actuation by slip control electronics may be switched into an open position in which pressure medium will be tapped from the wheel brakes, thereby the effective braking pressure being reduced.
In the connection between the pressure chamber of the hydraulic power booster and the rear axle wheel brakes of the automotive vehicle which are connected to the pressure chamber corresponding valve means are also arranged by means of which the pressure maybe kept constant at the wheel brakes associated with the rear axle while, and by means of a suitable switching-over of a solenoid valve a pressure reduction may be brought about.
The slip control electronics, in combination with speed sensors associated with the vehicle wheels, will register critical slip values at the vehicle wheels and release corresponding control signals to the solenoid valves. By a suitable actuation of the electromagnetically operable valves the braking pressure may be kept constant independently of the pressure generated in the braking pressure generator or may be reduced so as to effectively prevent an imminent state of lock-up during a braking action.
It is known that the occurrence of critical slip values is not limited to braking actions alone. It is possible that the driven wheels of the automotive vehicle may reach critical slip values when starting. The result thereof will be that acceleration of the vehicle will not be achieved. Further, in the case of rear-axle-driven automotive vehicles, there exists the danger that, with the rear wheels spinning, the directional stability of the automotive vehicle will be lost which may lead to critical driving situations. This is particular the case on road surfaces with small coefficients of friction.
It is thus an object of the present invention to further develop a hydraulic brake system of the type described above such as to achieve limitation of the acceleration slip down to uncritical values by means of structurally simple measures.